In order to facilitate automated video surveillance systems, it is advantageous to have the capability of performing scene change analysis, that is, analysis of the changes in a visible scene from one point in time to another. In particular, it is useful to be able to detect a change in the objects present in a given scene. For example, in comparing two images taken of the same scene at different times, the goal is to detect if an object has been added or removed from the scene. If the two images are obtained from the same viewpoint, detecting the presence of a change itself is often straightforward: when overall viewing conditions are constant, image differencing and thresholding will reveal regions in the scene that have undergone visible change in the two images. FIG. 1 is a diagram of a system which implements this technique. Images 1 and 2 are subtracted and thresholded at some threshold h, resulting in R, an image of the "change regions". Optional low-pass filtering and morphology may also be used to smooth the resulting image. For low-pass filtering, see Dana H. Ballard and Christopher M. Brown, Computer Vision. Prentice-Hall, Inc. Englewood Cliffs, N.J., 1982. For morphology see, Robert M. Haralick and Linda G. Shapiro. Computer and Robot Vision, volume 2. Addison-Wesley, Reading, Mass., 1993.
However, such a technique only determines the occurrence, location, and shape of change regions; it cannot, in itself, determine whether a detected change is due to an object that is present in the first image but absent in the second or vice-versa. It is an object of this invention to make such a determination in order to aid scene change analysis.